A turning mirror is a structure that is capable of coupling an optical signal, for example entering or coming out of an optical waveguide, with an optical transmitter or receiver, such as a laser diode or a photodetector. Generally, the two devices, i.e. the waveguide and the optical transmitter/receiver, do not lie on the same plane and therefore the turning mirror redirects the optical signal, reflecting the same of the angle needed. As an example, the mirror can make an abrupt 90° change of the direction of the light propagating in the optical waveguide such as to deflect the light toward the receiver.
Turning mirrors are extremely useful in integrated optical devices for directly processing optical signals; indeed such devices have become of greater importance as optical fiber communications are more and more widely used. Typical optical circuits applications require passive as well as active devices. Passive devices are formed within conventional silica optical circuits, whilst active devices, for example, are optical devices detectors and transmitters as well as modulators. The two different devices are interconnected through optical waveguides. As said, the turning mirror reflects signals coming from the waveguide linked to a passive device to the active device and vice-versa.
A general description of possible configurations and applications of turning mirrors in optoelectronic integrated circuits is reported for example in the U.S. Pat. No. 4,904,036 in the name of American Telephone and Telegraph Company and AT&T Bell Laboratories.
In U.S. Pat. No. 5,135,605 and U.S. Pat. No. 5,182,787 in the name of AT&T Bell Laboratories, a method to form an optical waveguide structure comprising a turning mirror realized by etching, and a method to form the same, are disclosed. In these patents, the mirror is realized employing a selective wet etching process to make a cavity in a waveguide, thus intercepting the light path propagating by the waveguide. Preferably, the cavity is made to be asymmetric with the side of the cavity remote from the waveguide sloping at typically 45° angle. The reflecting site of the cavity could be metallized to improve the reflectivity. The angle formed by the mirror depends on the etching process parameters.
Applicants have noted that this etching method to fabricate turning mirrors has a limited flexibility, in particular it is not suitable in case a plurality of turning mirrors having different angles is desired to be obtained on the same optical device.
Another turning mirror fabricating process is described in JP 6265738 in the name of Nippon Telegr & Teleph, in which many reflecting mirrors are formed simultaneously in optical waveguides. A photoresist is patterned using a photomask partly having mask patterns of the size or density of apertures gradually increasing in the longitudinal direction of optical waveguides. This patterned photoresist with sloped structures is used as an etching mask and the sloped structure is etched in the underlying layer film.
The method of resist exposure used in the process outlined in the above mentioned Japanese document is generally known as gray-tone or gray-scale lithography. Gray-scale lithography utilizes locally modulated exposure doses to develop the desired three-dimensional (3D) structure in the photoresist. Differential exposure doses lead to multiple depths of exposed photoresist across the surface. This is due to the ultraviolet light energy being absorbed by the photoactive compound as it travels in the depth of the photoresist. From the differential exposure doses, a gradient height photoresist structure corresponding to the designed silicon structure will remain once developed.
In “Investigation of gray-scale technology for large area 3D silicon MEMS structures”, written by C. M. Waits et al., published in J. Micromech. Microeng. 13 (2003), pages 170-177, it is shown that micromachining arbitrary 3D silicon structures for micro-electromechanical systems can be accomplished using gray-scale lithography along with dry anisotropic etching. Two important design limitations of gray-scale lithography have been investigated: the minimum usable pixel size and maximum usable pitch size. The mask used in the experiments performed in this article is a chrome-on-glass mask.
In “Cost-effective mass fabrication of multilevel diffractive optical elements by use of a single optical exposure with a gray-scale mask on high energy beam-sensitive glass”, written by W. Däschner et al., published in Applied Optics, Vol. 36 no 20 (1997), pages 4675-4680, a method for reproducing diffractive optical elements in quantity is described. A single e-beam writing step without any resist processing generates the mask. This single mask then contains all the necessary information previously contained in a set of five binary masks. In particular, HEBS glass is used to generate the gray-scale mask. This mask is then used in a contact aligner. The translation of electron dosage that was represented in the gray-scale mask by the variation of the optical density into a surface profile in the photoresist occurs. The photoresist chosen and used in these experiments is a positive novolac-resist.
Applicants note that gray-tone optical lithography for the definition of resist mask 3D profiles can permit a high flexibility. By using this technique, a wide range of 3D shapes, such as cavities with angled surfaces, can be achieved on a single optoelectronic integrated circuit. Unfortunately, grey-scale masks on optical photomasks, such as high-energy beam-sensitive glass (HEBS), are complicated to realize and generally very expensive.
An alternative method to obtain 3D shapes is to use variable-dose (or multi-dose) electron-beam lithography for the definition of the resist mask profile. The variable-dose e-beam lithography applied to the realization of complex structures is a topic addressed by several authors.
In “Free 3D Shaping with Gray-Tone Lithography and Multidose E-Beam Writing”, written by M. Kalus et al., published in Microelectronic Engineering 41/42 (1998), pages 461-464, 3D structures are realized by gray-tone lithography and directly by e-beam writing in a multiple dose regime. In particular, multidose e-beam writing assigns each element of a pattern, where a different resist height is proposed, to a precalculated local dose. The entire pattern is therefore split into layers which represent equal heights (like contour lines). A program transfers the three-dimensional design into a two-dimensional pattern. The correlation between the height representing layers and the local dose numbers is calculated. A 2 μm thick PMMA layer developed by a methyl-isobutyl-ketone ethanol 2:1 mixture for 60 s revealed a contrast of about γ=4.5.
European patent application n. 0651266 in the name of AT&T Corp. discloses a method of forming arbitrary angle mirrors in substrates. An erodible material, such as a photoresist, is applied to a substrate at a site and is exposed to radiation at that site which has a linear variation in energy at the surface of the erodible material. Due to this variation in exposure energy, a taper results in the erodible material after development. The tapered region is then etched in a manner which etches both the erodible material and the underlying substrate. In an embodiment of the invention, the tapered surface is obtained using e-beam lithography. A suitable programmed e-beam writer is scanned along the width of the intended grooved region. A number of scanning passes are made across the width of the region, where, after each pass, the beam is indexed along the length of the intended growth region. In this manner the full area of the intended growth region is sequentially exposed. As the e-beam is indexed along the length of the region, the energy of the e-beam is varied as required to alter the exposure depth to provide the desired taper.
From the cited prior art, it is shown that the fabrication of a turning mirror structure making use of multi-dose e-beam lithography to define the resist mask mirror profile and of reactive ion etching to transfer the mirror profile into the underlying substrate is possible. These steps however put some constraints on the e-beam process parameters. Applicants have noted that the resist thickness deposited on the substrate must be sufficiently high to allow the subsequent substrate etching, said thickness depending also on the selectivity during etching (with selectivity it is meant the etching rate ratio between the two materials constituting the mask and the substrate). Provided that for the two materials there is some etching recipe giving an optimal result and a maximum selectivity, it also exists a minimum resist thickness required for that recipe and for the substrate thickness to etch. Typically, the resists commonly used for e-beam lithography can reach thickness of only a few microns.
In the “SPIE Handbook of Microlithography, Micromachining and Microfabrication”, Volume I: Microlithography, Chapter 2.7.2.5 “Photoresists as e-beam resist”, it is written that some photoresists can be exposed by e-beam, although the chemistry is quite different from that of UV exposure. Because electrons cause both positive exposure and cross-linking at the same time, a photoresist film exposed with electrons must be developed with a “strong” developer for “positive” behavior, or, the same film can be blanket-exposed with UV light and then developed in a “weak” developer for “negative” behavior. An example is given: photoresist AZ5206 has a contrast of 4.
In “Exposure characteristics and three-dimensional profiling of SU8C resist using electron beam lithography”, written by W. H. Wong et al, and published in J. Vac. Sci. Technol. B 19(3) (2001), pages 732-735, the properties of a new type of chemically amplified resist, SU8C, are evaluated for electron beam lithography. The resist is a modification of the ultraviolet sensitive negative epoxy SU8. Experimental results show that sensitivity of SU8C is one of the highest among the different kinds of commercially available resists. The contrast, γ, can be adjusted to near unity by adjusting the postannealing time. Vertical resolution down to 20 nm has been obtained.
In “Gray scale structures formation in SU-8 with e-beam and UV”, written by V. Kudryashov et al. and published in Microelectronic Engineering 67-68 (2003), pages 306-311, an experimental study of the possibility to fabricate grey scale optical elements and 3D structures in SU-8 resist was carried out. It was found that the negative CAR SU-8 has a contrast smaller than 1 for PEB at temperature of 20-45° C. and sensitivity of the order of 1 μC/cm3 to 365-nm UV radiation. Continuous surface relief 3D structures formation in SU-8 by UV exposure was demonstrated both with true grey scale photomasks made with a high-energy beam sensitive glass and binary coded grey scale photomasks. A new technology for 3D self supporting structure formation in thick SU-8 resist layer was suggested, the techniques includes anchor elements exposure with UV for the whole resist thickness and a subsequent exposure of self-supporting fine structures in the upper resist layer only with a low-energy electron beam. International patent application WO 03/071587, in the name of University of Delaware, shows a process for making photonic crystal circuit and a photonic crystal circuit consisting of regularly-distributed holes in a high index dielectric material, and controllably-placed defects within this lattice, creating waveguides, cavities, etc for photonic devices. The process is based upon the finding that some positive ultraviolet (UV) photoresists are electron beam sensitive and behave like negative electron beam photoresists. This permits creation of photonic crystal circuits using a combination of electron beam and UV exposures. As a result, the process combines the best features of the two exposure methods: the high speed of UV exposure and the high resolution and control of electron beam exposure. As an example of suitable photoresist for the method disclosed in this patent, the AZ 5200 class photoresist is mentioned.
Applicants have noted that photoresists belonging to the AZ 5200 class have generally a high contrast, normally above 3. As an example, the AZ5206 photoresist has a contrast equal to 4 as indicated in the article written by W. H. Wong and quoted above (see page 733 of the article in issue).
Stitching errors are a common problem in e-beam lithography. An e-beam writing field (or exposure field) is the maximum deflection allowable for the scanning electron beam. A pattern with less than the exposure field can be written just deflecting the beam, whereas a pattern exceeding the exposure field must be written moving the stage where the substrate is mounted and by stitching the exposure fields together. This method is the origin of the so-called stitching error, which is due to the stage positioning accuracy limited by mechanical precision and which is not perfectly corrected by Beam Error Feedback (BEF) methods.
In “Fabrication of electron beam generated, chirped phase mask (1070.11-1070.66 nm) for fiber Bragg grating dispersion compensator” written by R. C. Tiberio et al., published in J. Vac. Sci. Technol. B 16(6) (1998) pages 3237-3240, the fabrication of a chirped, phase mask that was used to create a fiber Bragg grating (FBG) device for the compensation of chromatic dispersion in longhaul optical transmission networks is reported. Special attention has been paid to minimize any stitching error and exposure artifacts. This was done by using overlapping fields in a “voting” method. As a result, each grating line is exposed by the accumulation of three overlapping exposures at ⅓ dose. This translates any abrupt stitching errors into a small but uniform change in the line-to-space ratio of the grating.
In “Minimization of Phase Errors in Long Fiber Bragg Grating Phase Masks Made Using Electron Beam Lithography” written by J. Albert et al., published in Photonics Technology Letters, Vol. 8, no 10 (1996), pages 1334-1336, it is reported that centimeter-long fiber Bragg grating phase masks having several thousand periods are fabricated using electron beam lithography and require the stitching together of many electron beam writing fields. Among other, a technique to minimize the effect of phase errors arising from the stitching process is used. This technique consists of spreading the stitching error over the length of the mask by overlapping the electron-beam writing fields. The entire phase-mask is overwritten several times. For each “pass”, a different but constant field size is chosen. The field boundaries are chosen so that those of one pass do not overlap those of any other pass, thereby averaging out the stitching errors. The electron dose of each pass is adjusted so that the exposure dose of each grating line is optimal.